The efficient recovery of subterranean hydrocarbons such as oil and gas is assisted by obtaining reliable data about the physical conditions in a formation of interest. For example, a target formation typically includes hydrocarbon fluids that are under high pressure. Accurately measuring the formation pressure where such pressurized materials reside promotes safe and cost-effective operations in nearly all phases of hydrocarbon recovery. However, techniques for measuring formation pressure must overcome a number of technical challenges. One obstacle to pressure measurement is the mudcake that drilling mud tends to deposit on the wall of the wellbore.
A wellbore is typically filled with a drilling fluid such as water or a water-based or oil-based drilling fluid. The density of the drilling fluid is usually increased by adding certain types of solids that are suspended in solution. Drilling fluids containing solids are often referred to as drilling muds. The drilling fluids cool and lubricate the drill bit and carry the cuttings uphole to the surface. The solids in drilling fluids also increase the hydrostatic pressure of the wellbore fluids. By selecting drilling fluids weighted to a particular density, the column of drilling fluids creates a pressure downhole, which is greater than the pressure of the fluids in the formation. When the drilling fluid pressure is greater than the formation fluid pressure, the well is said to be in an over balanced condition. Conversely, if the formation pressure is greater than the fluid column, then the well is said to be in an under balanced condition. Control of formation fluids flowing into the well under high pressure minimizes the risk of a well blowout.
While an over balanced condition prevents well blowouts, it also has disadvantages, such as increased drilling costs due to slower penetration into the formation. Drilling fluid pressure in excess of formation pressure slows the penetration of the drill bit into the formation. In certain well environments it is preferred to maintain a neutral or slightly under balanced condition so as to achieve drilling speeds faster than those achieved while drilling in an over balanced condition. Drilling Practices Manual, Preston Moore, P. 18–22 Pennwell Publishing, 1974. Consequently, it is desirable to maintain a neutral balance or a slightly under balanced condition to maximize drilling penetration into the formation.
Drilling fluids create a mudcake as they flow into a formation by depositing solids on the inner wall of the wellbore. The mudcake on the wall of the wellbore tends to act like a filter and tends to isolate the high-pressure fluids of the wellbore from the relatively lower pressures of the formation. The mudcake helps prevent excessive loss of drilling fluid into the formation. The static pressure in the wellbore and the surrounding formation is typically referred to as hydrostatic pressure. Pressure in the formation beyond the mudcake gradually tapers off with increasing radial distance outward from the wellbore.
The measurement of formation pressures during drilling operations assists in locating strata most likely to produce hydrocarbons efficiently. Typically after the borehole is drilled, the well is logged by lowering a package of sensors downhole that gather data about the formation. Pressure data is useful in judging when a formation contains hydrocarbons and when such a formation may economically produce hydrocarbons. Often a wellbore may pass through more than one hydrocarbon-bearing formation, and formation pressure data assists the drilling engineer in determining whether to halt or continue drilling.
Further, the ability to monitor formation pressure during drilling is important to the desired practice of continuously adjusting the drilling mud density. This facilitates drilling through the maximum amount of formation in the shortest amount of time
To maintain the proper condition during drilling, whether neutral, over balanced or under balanced, it is necessary to measure the pressure of the formation fluids at the vicinity of the drill bit. However, the dynamic environment near the drill bit makes measurement of the formation fluids particularly difficult during logging while drilling (LWD) operations. In addition, the mudcake that forms on the wall of the borehole presents a further difficulty in determining formation fluid pressure at the bit during drilling. This mudcake forms a relatively non-permeable barrier between the instrument on the one side and the formation fluids on the other. The mudcake barrier hinders accurate measurement of the pressure of the formation fluids.
Prior art sensors are generally not capable of measuring formation fluid pressure during drilling. Consequently, rig personnel must closely monitor the drilling fluids flowing from the borehole for signs of increased formation fluid pressure. This often entails temporarily halting the drilling operation to allow pressure measurement of the formation. Once the drilling fluids show evidence of formation fluids flowing up the borehole, drilling is stopped and corrective measures are taken. However, this approach has particular drawbacks; and, it would be desirable to determine formation fluid pressure at the bit during drilling.
One such prior art instrument is a reservoir description tool (RDT) such as that disclosed in U.S. Pat. No. 5,644,076 (the '076 patent) entitled “Wireline Formation Tester Supercharge Correction Method”, incorporated herein by reference in its entirety. The RDT of the '076 patent includes a pressure sensing element mounted within a chamber of a housing having a piston to create a vacuum within the housing chamber. Hydraulic pads force the housing against the borehole wall; and, as the piston retracts to create a pressure reduction, a drawdown pressure removes the mudcake lining from the borehole wall. Fluids in the formation then enter the housing chamber allowing the pressure-sensing element to take a pressure reading. This tool allows only stationary measurements because drawdown pressure requires a tight seal between the housing and the borehole wall. This is undesirable because, aside from being time consuming, stationary measurements provide only discrete data points, not a continuous log. The drawback to discrete data points is that the fluid pressure between the discrete data points may vary dramatically and unpredictably.
Another borehole tool for removing the mudcake to measure the pressure of the formation fluids is disclosed in U.S. Pat. No. 5,969,241 (the '241 borehole tool) incorporated herein by reference. The '241 borehole tool measures pressure from within the borehole. A portion of the borehole wall is isolated from the surrounding borehole fluids by placing the chamber of the '241 borehole tool against the borehole wall. The chamber comprises a recess in an exterior surface of the '241 borehole tool. This patent describes an acoustic horn as the mechanism by which to excite fluids in a chamber. The mudcake present on the isolated portion of the borehole wall is disintegrated by an ultrasonic transducer, actuated by a piezoelectric stack, housed within the chamber. A pressure gauge then measures the pressure of the chamber to indicate the pressure of the earth formation.
Such a prior art tool also has deficiencies. For example, this borehole tool is inefficient because its vibrational energy does not transfer directly to the fluid. The vibrating horn is limited in the efficiency by which it transfers electrical energy to acoustical wave energy. Excitation of the piezoelectric stack creates a longitudinal wave resonance within the ultrasonic transducer. As the ultrasonic transducer resonates longitudinally, the vibrational energy is transferred to the fluid. However, the mechanical coupling of the ultrasonic transducer to the fluid is poor, thus much of the vibrational energy imparted by the piezoelectric stack remains in the ultrasonic transducer. This inefficient energy transfer is expected to reduce the vibrational energy available to break down the mudcake. Further, such tools are not compact and are not easily installed in the drill string, which must pass through the confined area of the borehole.
Notwithstanding the foregoing described prior art, there remains a need for a device that possesses the features of efficiently transferring vibrational energy to create a focused wave discharge that may be used to remove mudcake from a borehole wall. Further, it is desired that such a device may be utilized so as to minimize any interruption to the drilling process. It is also desired that such a tool be capable of use on different down hole assemblies such as wire line operations and near the drill bit in drilling operations. Additionally, the tool should be able to take pressure measurements on a continuous or near-continuous basis as the drill string descends the well bore.